1. Technical Field of the Invention
The present invention relates to testing equipment for telecommunications equipment. More particularly, and not by way of any limitation, the present invention is directed to an AC power fault machine capable of testing telecom line cards to known power fault immunity criteria.
2. Description of Related Art
Telecommunications (telecom) equipment deployed in today""s networks is required to comply with various governmental and industry standards not only to ensure seamless interoperability which reduces the risk of service interruption resulting from third-party product failures, but also to address various product safety issues. Accordingly, equipment manufacturers test their products to telecom industry standards commonly known as BellCore specifications (also sometimes referred to as Telcordia(trademark) specifications) which define an extensive list of electromagnetic compliance (EMC), product safety, and environmental requirements.
The BellCore specifications comprise two sets of testing standards, GR-1089-CORE and GR-63-CORE. The tests in GR-1089-CORE deal primarily with electrical phenomena, whereas the tests in GR-63-CORE are predominantly environmental in nature. While each set of standards is quite extensive, typically only a subset of the tests are required based on the type of equipment and its intended operating environment. Together, these two sets of standards specify the electrical and environmental requirements that networking hardware must meet in order to be located in a telco building, e.g., the telco""s central office (CO).
Besides the testing requirements, which are determined by product type, BellCore has defined additional testing levels generally referred to in the telecom industry as Telcordia(trademark)""s Network Equipment Building Systems (NEBS) levels. The appropriate NEBS level for a particular equipment is determined, again, by its intended operating environment and specific requirements of the Regional Bell Operating Companies (RBOCs). Generally, a higher NEBS level indicates a more stringent testing specification.
NEBS testing verifies that telecom equipment can operate successfully under certain electrical and physical environmental stresses and not pose a safety hazard to personnel and users. These stresses and hazards include earthquakes, airborne contaminants, fire and smoke, electromagnetic interference (EMI), electrical safety, and grounding.
Requirements under the three NEBS levels may be summarized as follows: Level 1 includes: electrical safety; lighting and AC power fault (2nd level); bonding and grounding; emissions; and fire resistance; Level 2 includes: all of Level 1 in addition toxe2x80x94electrostatic discharge (ESD) under normal operation; emissions and immunity; lighting and AC power fault (1st level); ambient temperature and humidity (operating); earthquake Zone 2 and office vibration; and airborne contaminants (indoor level); Level 3 includes: all of Level 1 and Level 2 in addition toxe2x80x94ESD (installation and repair); open door emissions and immunity; ambient temperature and humidity (short-term); earthquake Zone 4; airborne contaminants (outdoor level); and transportation and handling. Each test within these three Levels is defined in either the GR-1089-CORE or GR-63-CORE documentation.
Testing of telecom ports, i.e., tip-and-ring (T and R) interfaces of the line cards utilized in telecom equipment, for lightning and AC power fault immunity in accordance with the above-referenced standards is necessary for several reasons. Power companies and the Local Exchange Carriers (LECs) often serve the same customers, and frequently employ joint-use facilities such as supporting structures or a common trench for their respective outside plant. Metallic conductors, such as cable or wire pairs serving telecom equipment may be exposed to electrical surges resulting from lightning and commercial power system disturbances. Despite the presence of protective devices in the telecommunications network that limit the effect of lightning and power surges, a portion of these disturbances can be impressed on the network equipment. Accordingly, under abnormal conditions, for instance, the power and telecommunications lines may come into electrical contact. If the contact occurs to a primary power line, faults may be cleared quickly by the power system (5 seconds or less), and protectors (e.g., carbon blocks) can limit 60 Hz voltages appearing on the T or R conductors to maximum of approximately 600 VRMS with respect to ground. If the contact occurs to a secondary power line, the full secondary voltage with respect to ground (up to about 275 VRMS in some cases) may appear on the T and R conductors, which may persist indefinitely as the secondary fault may not be cleared by the power system.
Moreover, because electric power lines and telecom lines often occupy parallel routes as a result of a common right-of-way, the magnetic field produced by currents in a nearby power line, especially under abnormal conditions such as a phase-to-ground fault, may result in large voltages being induced into the telecom lines through electromagnetic coupling. The induced voltages appear longitudinally in the T and R conductors and may approach several hundred volts. Lower levels of induction may result from a high-impedance power fault such as a phase conductor falling to the earth. If the resulting unbalanced current is within the normal operating range of the power system, or if power system breakers or fuses do not operate, the fault may persist for an extended period of time.
Under the BellCore""s GR-1089-CORE standard, the lightning surge and AC power fault immunity criteria include compliance with various tests such as short-circuit tests (tip to ring, tip to ground with ring open-circuited, ring to ground with tip open-circuited, tip and ring to ground simultaneously, et cetera) and several AC power fault tests. As set forth hereinabove, these criteria are separated into 1st level and 2nd level criteria. To comply with the 1st level criteria, it is required that the telecom equipment under test (i.e., EUT) be undamaged and continue to operate properly after power stress is removed. To comply with the 2nd level criteria, the EUT may sustain damage, but it is required that the equipment not become a fire, fragmentation (that is, forceful ejection of fragments), or an electrical safety hazard.
While several lightning machines are available for conducting the lightning compliance tests required under the BellCore standards alluded to hereinabove, there is a paucity of appropriate AC power fault (PF) machines capable of sourcing power to telecom units under test for adequately conducting the AC power fault compliance tests. Further, the relatively few solutions extant today are beset with various shortcomings and drawbacks. First, the existing AC power fault machines are typically custom-designed to a large extent and, accordingly, incapable of accommodating various telecom equipment types and form factors. Additionally, these machines are quite expensive to manufacture owing at least in part to their custom design. In spite of the custom design, however, the existing PF machines are not capable of providing power safely to the EUT to conduct power fault tests at higher voltages as required under the relevant BellCore standards. Furthermore, the conventional PF machines are too bulky for portable testing applications, thereby requiring an extensive (and expensive) dedicated laboratory facility.
Accordingly, the present invention advantageously provides a portable yet versatile power fault testing apparatus for testing telecommunications equipment""s line cards to BellCore""s 1st Level AC power fault standards in low voltage as well as high voltage ranges. In the presently preferred exemplary embodiment of the present invention, a switched transformer unit is coupled to a single-phase AC line voltage source via a circuit breaker, isolation transformer and an autotransformer. The switched transformer unit is operable to generate a switched voltage output in 0-220 volt range or in 0-1100 volt range. A primary control relay is coupled to the switched transformer unit for applying the switched voltage output for selected time durations under control of an external computer which is interfaced via suitable interface electronics circuitry. Preferably, an AC-to-DC converter is included in the apparatus for providing DC power to the interface electronics.
A test selection switch coupled to the primary control relay connects the switched voltage output to either a resistive test circuit portion or an inductive test circuit portion, each of which is designed to comply with the respective AC power fault testing requirements.
The resistive test circuit portion preferably includes four identical test circuit branches to couple to either two-wire or four-wire T/R interfaces of a line card. Each test circuit branch is comprised of a plurality of resistors and a pair of relays whose operation is controlled by a three-position switching circuit. Preferably, six resistors of equal value (e.g., 200 ohms) are coupled in a predetermined network configuration such that the three positions of the switching circuit are operable to provide an equivalent resistance of 150 ohms, 600 ohms or 1000 ohms on each branch.
The inductive test circuit portion includes an inductive test transformer whose primary coil side is operable to be connected to the switched voltage output via the test selection switch. The secondary coil side is comprised of two secondary coils that provide two output voltage nodes operable to be coupled to the T/R interface of the line card under test. A plurality of capacitors are disposed between the two output voltage nodes as well as between each voltage node and the ground.